Method of separating the fractions of a gaseous mixture in a gas rectifying system



Jan. 21, 1958 H. FOKKER ETAL 2,320,352

METHOD OF SEPARATING THE FRACTIONS 0F AGASEOUS MIXTURE IN A GAS RECTIFYING SYSTEM Filed Nov. 8, 1954 2 Sheets-Sheet 1 INVENTORS HERMAN FOK KER JACOB WILLEM LAURENS Kl-ILER HERRE mum Jan. 21-, 1958 H. FOKKER ETAL 2,820,352

METHOD OF SEPARATING THE FRACTIONS OF A GASEOUS MIXTURE IN A GAS RECTIFYING SYSTEM Filed NOV. 8,' 1954 2 Sheets-Sheet 2 -INVENTOR5 HERMAN FOKKER JACOB WIL'LEM LAURENS KOHLER ERRE NIA NIETHOD OF SEPARATING THE FRACTIONS 013 st gAEBI'OUS MIXTURE IN A GAS RECTIFYING Herman Fokker, Jacob Willem Laurens Kiihler, and Herre Rinia, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application November 8, 1954, Serial No. 467,558

Claims priority, application Netherlands November 7, 1953 5 Claims. (Cl. 62-123) The invention relates to a method of separating a gaseous mixture in a gas rectifying system comprising a gas restifying column operating under substantially atmospheric pressure. The medium used in the system withdraws heat from the upper portion of the column and supplies heat to the liquid contained in the boiling vessel associated with the column. The cycle of the medium includes compression and subsequent cooling by thermal contact with the liquid in the boiling vessel and withdrawal of heat from the column subsequent to a reduction of pressure.

Known systems in which said method is carried out often have a closed circuitin which a medium performs a thermodynamic cycle. The medium is compressed and then cooled by thermal contact with the fraction having the highest boiling point which is contained in the boiling vessel associated with the gas rectifying column whereupon this fraction is evaporated. The medium then expands and withdraws heat from the column, which is often carried out in a separate condenser. Then the medium flows back to the compressor. It is common practice to heat the medium flowing back to the compressor to about room temperature by thermal contact with the compressed medium. If thermal contact is omitted, the compressor operates at a very low temperature.

In both cases it is necessary to withdraw a quantity of heat from the gas rectifying system in addition to the amount withdrawn by the cycle of the medium. This may be carried out in various ways and in the known systems this may be done for example by successively compressing and expanding the medium associated with the gas rectifying system.

As an alternative, at least part of the fraction having the lower boiling point could be condensed directly by means of a cold-gas refrigerator. The term cold-gas refrigerator is to be understood to mean herein a cooling machine operating on the reversed hot-gas reciprocating engine principle. These machines may be constructed in various ways, for example as displacer-piston machines, double-acting machines, machines in which two cylinders are at an angle to one another, or as a machine of which the working space is combined with that of a hot-gas motor. These machines permit of obtaining very low temperature of for example 200 C. in one step.

According to the present invention it has been found that the efficiency of the gas rectifying system can be increased, if the cold-gas refrigerator withdraws heat from the gas rectifying system at a point dilfering from that referred to above, so that the heat is not withdrawn at the temperature of the fraction having the lower boiling point.

According to the invention heat is withdrawn by a cold gas refrigerator from at least part of the medium at approximately the boiling temperature of the higher Patented Jan. 21, 1:958

boiling fraction of the mixture to be separated after the medium has been compressed but before its pressure has been reduced.

Although in this method the quantity of medium to be compressed by the compressor exceeds the quantity in the method described above, it has been found the efliciency of the system will finally increase due to the fact that the eiliciency and the cold production of the cold-gas refrigerator increase considerably since the machine is required to bridge a smaller temperature dilference in one step. It has been found that a rather important increase in output is obtained.

As described above, the medium may perform a closed cycle. However, if air is separated into fractions and if the medium is nitrogen, the latter may be supplied to the column at a low temperature, for example at 196 C., in one embodiment of the invention, in which heat is withdrawn from the column. A greater quantity by weight of nitrogen is withdrawn from the column than is supplied by the medium. Part of this quantity is compressed and subsequently supplied to the column, while the remaining quantity is withdrawn from the system.

The aforesaid methods may be carried out successfully if the fraction having the higher boiling point is obtained in the liquid state.

The device for carrying out the methods described above comprises a gas rectifying column including means to withdraw heat therefrom, a compressor to increase the pressure of this medium, an expansion device for reducing the pressure of the medium, and with a cold-gas refrigerator located in the duct system between the compressor and the expansion device which withdraws heat from at least part of the medium.

According to a further embodiment of the invention the column is constructed as a single column. A single column is to be understood to mean herein a column in which the fraction having the lower boiling point is withdrawn from the upper portion thereof, while the gaseous mixture to be separated is supplied to the column at an area lying between the upper and lower portions thereof.

The invention will be described with reference to the accompanying drawing in which:

Fig. l is a diagrammatic representation of a device for separating a gaseous mixture including a cold-gas refrigerator which withdraws heat from the medium after it has been in thermal contact with the fraction of the mixture having the higher boiling point;

Fig. 2 is another embodiment of the invention showing a device in which a cold-gas refrigerator cools the medium prior to making thermal contact with the fraction having the higher boiling point, wherein the compressor operates at low temperature; and

Fig. 3 is a further embodiment of the invention in which part of the medium is cooled by a cold-gas refrigerator and an additional quantity thereof of the medium is cooled by thermal contact with the fraction having the higher boiling point.

The gas rectifying system shown in Fig. 1 comprises a gas rectifying column 1 and a boiling vessel 2. In addition, the system comprises a compressor 3, in which a medium, for example, nitrogen, is compressed to about 5 atmospheres. The medium then flows through the duct 4 to a heat exchanger 5', in which its temperature is reduced, and through a duct 6 to a heat exchanger 7 situated in the boiling vessel 2 of the column. In the heat exchanger 7 the major portion of the medium condenses and then flows through a duct 8 to a cold-gas refrigerator which is driven by a motor 10. Heat is withdrawn from the medium by means of a cold-gas refrigerator 9. The medium then fiows through a duct 11 to an expansion device 12, in which its pressure is reduced to atmospheric or substantially atmospheric pressure, after which the medium is supplied to the column through a duct 13 The gaseous mixture to be separated, for example air, is drawn in through a compressor 14- and its pressure is increased to such a value that the resistance of the duct system between the compressor and the column can be overcome. The gaseous mixture, after being slightly compressed, flows through a duct 15 to the heat exchanger 5, in which it is cooled and thereafter flows through the duct 16 to a heat exchanger 17 located in the boiling vessel 2 of the column. The gaseous mixture is then supplied through a duct 13 to the single column 1. In this column the gaseous mixture is separated into fractions, the fraction having the highest boiling point, i. e. in this case the oxygen, flowing downward and being coi lected in the boiling vessel 2. The major portion of the liquid in the boiling vessel evaporates due to the supply of heat through the heat exchangers '7 and 1'7. The vapour thus produced rises in the column. fraction having the higher boiling point is withdrawn as a liquid through a duct 19 of the column. The fraction having the lower boiling point, for example, nitrogen, rises in the column, partially condenses in the upper part of the column due to the very low temperature of the medium which is supplied to the column through the duct 13. Part of this fraction and the evaporated medium escape from the column through the duct 20 to the heat exchanger 5, where the medium withdraws heat from the compressed medium in duct 4 to this heat exchanger and from the gaseous mixture to be separated. Part of the medium then fiows through a duct 21 to the compressor 3, and part of the medium escapes through a duct 22 from the system. In the embodiment described above air is separated into fractions. The temperature at the top of the column is in this case 196 C., whereas the temperature in the boiling vessel is -183 C. In spite of the fact that a larger quantity of medium is to be compressed when the cold gas refrigerator operates at approximately the temperature of the higher boiling fracpressor 30, which operates at low temperature in this embodiment. After compression the medium flows through a duct 6 to the cold-gas refrigerator 31, in which its temperature is reduced and wherein it may even be condensed, after which the medium is supplied through a duct 32 to the heat exchanger '7 in the boiling vessel 2 and through the ducts 11 and 13, provided with the expansion device 12, to a condenser 33 located in the upper portion of the column. In this condenser the medium evaporates, withdrawing heat from the column. The medium then flows through the duct 29 back to the compressor 30. In this embodiment the medium operates in a closed cycle. The gaseous mixture to be rectified is supplied through a compressor 14, flows through a duct 15 to a heat exchanger 34, where its temperature is reduced, and then flows through a duct 35 to the heat exchanger 17 and through the duct 18 to the column. The gaseous mixture is separated into fractions in the column. The fraction having the higher boiling point is collected in the boiling vessel 2 and withdrawn as a liquid through a duct 19 from the system. The fraction having the lower boiling point rises in the column, is partly condensed in the condenser 33, after which the condensate serves as a cleaning liquid in the column, whereas part of this gaseous fraction flows through a duct 36 to the heat exchanger 34, in which the gaseous mixture to be recti- Part of the fied is conducted away. The fraction having the lower boiling point is then conducted away from the system through a duct 37.

In the embodiment shown in Fig. 3 the elements correspending to those shown in Fig. l are designated by the same reference numerals. The medium is compressed in a compressor 3 and then flows through the duct 4 to the heat exchanger 5, in which it is cooled. Part of the medium is then supplied through the duct 6 to the heat exchanger 7 in which it condenses and supplies heat to the liquid contained in the boiling vessel. Some of the medium flows through a duct 40 to a cold-gas refrigerator 41, which condenses it and the condensate is supplied through a duct 42 to the duct 11, which communicates with the heat exchanger 7. As in the case shown in Fig. l, the medium flows through the duct 11, the expansion device 12 and the duct 13 to the single gas rectifying column. As in the construction shown in Fig. 1 the evaporated medium and the fraction having the lower boiling point, both of which preferably have the same composition, escape from the upper end of the column,. Then the medium flows through the duct 20 to the heat exchanger 5 and part of the medium is withdrawn from the system through the duct 22. The gaseous mixture to be separated is supplied through a compressor 14, the duct 15, the heat exchanger 5, the duct 16, the heat exchanger 17 and the duct 18 to the gas rectifying column.

The embodiments shown in Figs. 2 and 3 also provide an appreciable increase in output.

While we have shown and described the preferred embodiment of our invention, it will be understood that the latter may be embodied otherwise than as herein specifically illustrated or described and that in the illustrated embodiment certain changes in the details of construction and in the arrangement of parts may be made without departing from the underlying idea or principle of the invention within the scope of the appended claims.

What is claimed is:

1. A method of separating the fractions of a gaseous mixture in a gas rectifying system by thermal contact with a medium supplied to a gas rectifying column having a boiling vessel associated therewith comprising compressing said mixture, cooling said mixture, introducing said cooled mixture into said gas rectifying column at a point intermediate the ends thereof, compressing said medium, adding heat to the higher boiling fraction in said boiling vessel, expanding said medium, and cooling said medium at approximately the temperature of the higher boiling fraction after compression of the medium prior to expansion of the medium, the heat being withdrawn from at least part of said medium at approximately the temperature of the higher boiling fraction of said mixture by a cold gas refrigerator comprising a cylinder, two pistons operating in said cylinder with a constant phase difference and defining two chambers in which a closed thermodynamic cycle is performed by a gaseous medium of invariable chemical composition in one and the same state of aggregation, the volume of gaseous medium in said chambers varying continuously while one of said chambers has a low temperature and the other chamber has a higher temperature, the chambers being connected with one another through a cooler, regenerator and freezer, said cycle being performed independently of said fractionat ing process.

2. A method as claimed in claim 1 in which air is the gaseous mixture and nitrogen is the medium supplied to the column at a. low temperature to withdraw heat therefrom, removing said nitrogen from said column, and compressing a portion of said gas.

3. A method as claimed in claim 1 in which the fraction having the higher boiling point is obtained in the liquid state.

4. An apparatus for separating the fractions of a gaseous mixture in a gas rectifying system by thermal contact with a medium comprising a compressor to increase the pressure of said medium, a gas rectifying column, means for introducing said mixture to said column at a point intermediate the ends thereof, a boiling vessel associated with said column and containing the higher boiling fraction of said mixture, an expansion device for reducing the pressure of said medium, and a cold gas refrigerator located in said system but operating independently thereof between said compressor and said expansion device for removing heat from said medium at approximately the temperature of the higher boiling fraction in said boiling vessel, said cold gas refrigerator comprising a cylinder, two pistons operating in said cylinder with a constant phase difference and defining two chambers in which a closed thermodynamic cycle is performed by a gaseous medium of invariable chemical composition in one and the same state of aggregation, the volume of gaseous medium in said chambers varying continuously while one of said chambers has a low temperature and the other chamber has a higher temperature, a cooler, a regenerator and a freezer connecting said chambers with one another.

5. An apparatus as claimed in claim 4 in which said column is constructed as a single column.

References Cited in the file of this patent UNITED STATES PATENTS 2,284,662 Kahle June 2, 1942 2,386,297 Dennis Oct. 9, 1945 2,411,711 De Baufre Nov. 26, 1946 2,417,279 Van Nuys Mar. 11, 1947 2,423,273 Van Nuys July 1, 1947 2,608,070 Kapitza Aug. 26, 1952 2,620,637 Schilling Dec. 9, 1952 2,627,731 Benedict Feb. 10, 1953 FOREIGN PATENTS 675,347 Germany May 6, 1939 

1. A METHOD OF SEPARATING THE FRACTIONS OF A GASEOUS MIXTURE IN A GAS RECTIFYING SYSTEM BY THERMAL CONTACT WITH A MEDIUM SUPPLIED TO A GAS RECTIFYING COLUMN HAVING A BOILING VESSEL ASSOCIATED THEREWITH COMPRISING COMPRESSING SAID MIXTURE, COOLING SAID MIXTURE, INTRODUCING SAID COOLED MIXTURE INTO SAID GAS RECTIFYING COLUMN AT A POINT INTERMEDIATE THE ENDS THEREOF, COMPRESSING SAID MEDIUM, ADDING HEAT TO THE HIGHER BOILING FRACTION IN SAID BOILING VESSEL, EXPANDING SAID MEDIUM, AND COOLING SAID MEDIUM AT APPROXIMATELY THE TEMPERATURE OF THE HIGHER BOILING FRACTION AFTER COMPRESSION OF THE MEDIUM PRIOR TO EXPANSION OF THE MEDIUM, THE HEAT BEING WITHDRAWN FROM AT LEAST PART OF SAID MEDIUM AT APPROXIMATELY THE TEMPERATURE OF THE HIGHER BOILING FRACTION OF SAID MIXTURE BY A COLD GAS REFRIGERATOR COMPRISING A CYLINDER, TWO PISTONS OPERATING IN SAID CYLINDER WITH A CONSTANT PHASE DIFFERENCE AND DEFINING TWO CHAMBERS IN WHICH A CLOSED THERMODYNAMIC CYCLE IS PERFORMED BY A GASEOUS MEDIUM OF INVARIABLE CHEMICAL COMPOSITION IN ONE AND THE SAME STATE OF AGGREGATION, THE VOLUME OF GASEOUS MEDIUM IN SAID CHAMBERS VARYING CONTINUOUSLY WHILE ONE OF SAID CHAMBERS HAS A LOW TEMPERATURE AND THE OTHER CHAMBER HAS A HIGHER TEMPERATURE, THE CHAMBERS BEING CONNECTED WITH ONE ANOTHER THROUGH A COOLER, REGENERATOR AND FREEZER, SAID CYCLE BEING PERFORMED INDEPENDENTLY OF SAID FRACTIONATING PROCESS. 